// MultiTexture.js (c) 2012 matsuda and kanda
// Vertex shader program
var VSHADER_SOURCE = `
attribute vec4 a_Position;
attribute vec2 a_TexCoord;
varying vec2 v_TexCoord;
void main() {
  gl_Position = a_Position;
  v_TexCoord = a_TexCoord;
}
`;

// Fragment shader program
var FSHADER_SOURCE = `
  #ifdef GL_ES
  precision mediump float;
  #endif
  uniform sampler2D u_Sampler0;
  uniform sampler2D u_Sampler1;
  uniform sampler2D u_Sampler2;
  varying vec2 v_TexCoord;
  void main() {
    vec4 color0 = texture2D(u_Sampler0, v_TexCoord);
    vec4 color1 = texture2D(u_Sampler1, v_TexCoord);
    vec4 color2 = texture2D(u_Sampler2, v_TexCoord);
    gl_FragColor = color0 + color1 + color2;
  }
`;

function main() {
  // Retrieve <canvas> element
  var canvas = document.getElementById("webgl");

  // Get the rendering context for WebGL
  var gl = getWebGLContext(canvas);
  if (!gl) {
    console.log("Failed to get the rendering context for WebGL");
    return;
  }

  // Initialize shaders
  if (!initShaders(gl, VSHADER_SOURCE, FSHADER_SOURCE)) {
    console.log("Failed to intialize shaders.");
    return;
  }

  // Set the vertex information
  var n = initVertexBuffers(gl);
  if (n < 0) {
    console.log("Failed to set the vertex information");
    return;
  }

  // Specify the color for clearing <canvas>
  gl.clearColor(0.0, 0.0, 0.0, 1.0);

  // Set texture
  if (!initTextures(gl, n)) {
    console.log("Failed to intialize the texture.");
    return;
  }
}

function initVertexBuffers(gl) {
  var verticesTexCoords = new Float32Array([
    // Vertex coordinate, Texture coordinate
    -0.5,
    0.5,
    0.0,
    1.0,
    -0.5,
    -0.5,
    0.0,
    0.0,
    0.5,
    0.5,
    1.0,
    1.0,
    0.5,
    -0.5,
    1.0,
    0.0,
  ]);
  var n = 4; // The number of vertices

  // Create a buffer object
  var vertexTexCoordBuffer = gl.createBuffer();
  if (!vertexTexCoordBuffer) {
    console.log("Failed to create the buffer object");
    return -1;
  }

  // Write the positions of vertices to a vertex shader
  gl.bindBuffer(gl.ARRAY_BUFFER, vertexTexCoordBuffer);
  gl.bufferData(gl.ARRAY_BUFFER, verticesTexCoords, gl.STATIC_DRAW);

  var FSIZE = verticesTexCoords.BYTES_PER_ELEMENT;
  //Get the storage location of a_Position, assign and enable buffer
  var a_Position = gl.getAttribLocation(gl.program, "a_Position");
  if (a_Position < 0) {
    console.log("Failed to get the storage location of a_Position");
    return -1;
  }
  gl.vertexAttribPointer(a_Position, 2, gl.FLOAT, false, FSIZE * 4, 0);
  gl.enableVertexAttribArray(a_Position); // Enable the assignment of the buffer object

  // Get the storage location of a_TexCoord
  var a_TexCoord = gl.getAttribLocation(gl.program, "a_TexCoord");
  if (a_TexCoord < 0) {
    console.log("Failed to get the storage location of a_TexCoord");
    return -1;
  }
  gl.vertexAttribPointer(a_TexCoord, 2, gl.FLOAT, false, FSIZE * 4, FSIZE * 2);
  gl.enableVertexAttribArray(a_TexCoord); // Enable the buffer assignment

  return n;
}

function initTextures(gl, n) {
  // Create a texture object
  var texture0 = gl.createTexture();
  var texture1 = gl.createTexture();
  var texture2 = gl.createTexture();
  if (!texture0 || !texture1|| !texture2) {
    console.log("Failed to create the texture object");
    return false;
  }

  // Get the storage location of u_Sampler0 and u_Sampler1
  var u_Sampler0 = gl.getUniformLocation(gl.program, "u_Sampler0");
  var u_Sampler1 = gl.getUniformLocation(gl.program, "u_Sampler1");
  var u_Sampler2 = gl.getUniformLocation(gl.program, "u_Sampler2");

  if (!u_Sampler0 || !u_Sampler1 || !u_Sampler2) {
    console.log("Failed to get the storage location of u_Sampler");
    return false;
  }

  // Create the image object
  var image0 = new Image();
  var image1 = new Image();
  var image2 = new Image();

  if (!image0 || !image1 || !image2) {
    console.log("Failed to create the image object");
    return false;
  }
  // Register the event handler to be called when image loading is completed
  image0.onload = function () {
    loadTexture(gl, n, texture0, u_Sampler0, image0, 0);
  };
  
  image1.onload = function () {
    loadTexture(gl, n, texture1, u_Sampler1, image1, 1);
  };
  
  image2.onload = function () {
    loadTexture(gl, n, texture2, u_Sampler2, image2, 2);
  };
  // Tell the browser to load an Image
  image0.src = "../resources/blue.jpg";
  image1.src = "../resources/green.jpg";
  image2.src = "../resources/red.jpg";

  return true;
}
// Specify whether the texture unit is ready to use
var g_texUnit0 = false;
var g_texUnit1 = false;
var g_texUnit2 = false;
function loadTexture(gl, n, texture, u_Sampler, image, texUnit) {
  gl.pixelStorei(gl.UNPACK_FLIP_Y_WEBGL, 1); // Flip the image's y-axis
  // Make the texture unit active
  if (texUnit == 0) {
    gl.activeTexture(gl.TEXTURE0);
    g_texUnit0 = true;
  } else if (texUnit == 1) {
    gl.activeTexture(gl.TEXTURE1);
    g_texUnit1 = true;
  } else if (texUnit == 2) {
    gl.activeTexture(gl.TEXTURE2);
    g_texUnit2 = true;
  }
  // Bind the texture object to the target
  gl.bindTexture(gl.TEXTURE_2D, texture);

  gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MAG_FILTER, gl.NEAREST);
  gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MIN_FILTER, gl.NEAREST);

  // Set texture parameters
  // gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MIN_FILTER, gl.LINEAR);
  // Set the image to texture
  gl.texImage2D(gl.TEXTURE_2D, 0, gl.RGBA, gl.RGBA, gl.UNSIGNED_BYTE, image);
  
  gl.uniform1i(u_Sampler, texUnit); // Pass the texure unit to u_Sampler

  // Clear <canvas>
  gl.clear(gl.COLOR_BUFFER_BIT);

  if (g_texUnit0 && g_texUnit1 && g_texUnit2) {
    gl.drawArrays(gl.TRIANGLE_STRIP, 0, n); // Draw the rectangle
  }
}
